relax the parameter of ensureMove; allow let statements (#22466)

* relax the parameter of `ensureMove`; allow let statements

* fixes the test

(cherry picked from commit 4c89223171)

compiler/semmagic.nim

@@ -668,7 +668,8 @@ proc magicsAfterOverloadResolution(c: PContext, n: PNode,
       result.typ = expectedType # type inference for empty sequence # bug #21377
   of mEnsureMove:
     result = n
-    if isAssignable(c, n[1]) notin {arLValue, arLocalLValue}:
-      localError(c.config, n.info, "'" & $n[1] & "'" & " is not a mutable location; it cannot be moved")
+    if n[1].kind in {nkStmtListExpr, nkBlockExpr,
+              nkIfExpr, nkCaseStmt, nkTryStmt}:
+      localError(c.config, n.info, "Nested expressions cannot be moved: '" & $n[1] & "'")
   else:
     result = n

tests/system/tensuremove.nim

@@ -20,7 +20,8 @@ block:
     discard x.s
 
   proc main =
-    var x = X(s: "abcdefg")
+    let m = "abcdefg"
+    var x = X(s: ensureMove m)
     consume(ensureMove x)
 
   static: main()

tests/system/tensuremove2.nim

@@ -1,5 +1,5 @@
 discard """
-  errormsg: "'if true: s else: String()' is not a mutable location; it cannot be moved"
+  errormsg: "Nested expressions cannot be moved: 'if true: s else: String()'"
 """
 
 type